Motte and bailey (MAB) is a combination of bait-and-switch and equivocation in which someone switches between a "motte" (an easy-to-defend and often common-sense statement, such as "culture shapes our experiences") and a "bailey" (a hard-to-defend and more controversial statement, such as "cultural knowledge is just as valid as scientific knowledge") in order to defend a viewpoint. Someone will argue the easy-to-defend position (motte) temporarily, to ward off critics, while the less-defensible position (bailey) remains the desired belief, yet is never actually defended.

In short: instead of defending a weak position (the "bailey"), the arguer retreats to a strong position (the "motte"), while acting as though the positions are equivalent. When the motte has been accepted (or found impenetrable) by an opponent, the arguer continues to believe (and perhaps promote) the bailey.

Note that the MAB works only if the motte and the bailey are sufficiently similar (at least superficially) that one can switch between them while pretending that they are equivalent.

The term "motte and bailey" was created by Nicholas Shackel, a British professor of Philosophy. Shackel named it after the motte-and-bailey castle, in which a highly-protected stone-fortified keep (the motte) is accompanied by an enclosed courtyard protected by sharpened wooden palisades (the bailey).[1][2] Shackel used the phrase to criticize postmodernists who switch between arguing for uncontroversial statements, such as stating that culture influences a person's interpretation of the world (an easy-to-defend motte), and promoting highly controversial positions, e.g., that interpretations of the world based on religious mythology are as valid as scientific interpretations (an indefensible bailey). When the bailey (every-viewpoint-is-valid) is confronted, they often retreat to the motte (culture-shapes-our-experiences) and mock the critic for -- supposedly -- thinking this isn’t true.

A Motte and Bailey castle is a medieval system of defence in which a stone tower on a mound (the Motte) is surrounded by an area of pleasantly habitable land (the Bailey), which in turn is encompassed by some sort of a barrier, such as a ditch. Being dark and dank, the Motte is not a habitation of choice. The only reason for its existence is the desirability of the Bailey, which the combination of the Motte and ditch makes relatively easy to retain despite attack by marauders. When only lightly pressed, the ditch makes small numbers of attackers easy to defeat as they struggle across it: when heavily pressed the ditch is not defensible, and so neither is the Bailey. Rather, one retreats to the insalubrious but defensible, perhaps impregnable, Motte. Eventually the marauders give up, when one is well placed to reoccupy desirable land.

For my original purposes the desirable but only lightly defensible territory of the Motte and Bailey castle, that is to say, the Bailey, represents philosophical propositions with similar properties: desirable to their proponents but only lightly defensible. The Motte represents the defensible but undesired propositions to which one retreats when hard pressed. Shackel 2014[4][5]

Scott Alexander[3] (and related bloggers[6][7]) used the term to criticize two-faced proponents of social justice.

This method is an example of the "two-step", in which one repeatedly appears to concede weaker parts of an argument, then re-asserts the original claim, unaltered. The name refers to a quick and repetitious dance step. It relies on short attention span, short memories, and/or rapidly changing audience.

Clarifying one's views to exclude an incorrect, expansive interpretation is not a motte-and-bailey fallacy, provided that what you defend is a correct and intended interpretation of your earlier statements. The problem with the motte and bailey is that it represents a constantly shifting target: now easy, now hard.